The present invention relates to stationery ground antenna systems for fixed microwave service where narrow beam transmission from point to point is required and interference with other point to point microwave service is to be avoided. The invention also relates to ground to satellite transmission where a null in the radiation pattern must be placed at an adjacent stationery satellite.
Heretofore, the Federal Communications Commission (FCC) of the United States Government has set forth very strict beam width and side lobe requirements for fixed microwave service from point to point on the ground. In particular, the FCC has specified "Antenna limitations" for fixed point to point ground or ground to satellite microwave service employing directional microwave antennas for transmitting or receiving between one point and another wherein the radiation pattern of the transmitting antenna has a major lobe of radiation directed toward the receiving station at the other point with which it communicates or, if the path employs a passive repeater between the two points, to the center of the repeater reflector. The requirement is that the directional antenna meet performance standards for microwave frequencies in the range extending between 952 MHZ and 15,000 MHZ. These standards require of the transmitter antenna a maximum width to 3 dB points (beam width angle in degrees) and the minimum radiation suppression (in dB) that must be achieved at angles from the center line of the main beam. In other words, the standards insist that side lobes of the transmitting antenna beam pattern at given angles be suppressed.
Clearly, the FCC requirement for directional microwave antennas are intended to reduce possible interference between different point to point pairs of stations. Heretofore, the usual transmitting antenna has comprised a single horn or reflector (dish) and some effort has been made to reduce interference with other stations by making the size very large and/or by shrouding to narrow the beam width of this antenna and reduce side lobe levels. In many situations these efforts have not been satisfactory. Hence, it is one object of the present invention to provide a method and means of providing a microwave antenna system for point to point ground transmission, ground to satellite transmission whereby some of the above mentioned limitations of prior structures and techniques are avoided. It is a particular object to provide a microwave antenna system capable of meeting the above mentioned FCC requirements of maximum beam width in degrees to 3 dB points and minimum radiation suppression at specific angles.